I just used 5200 to seal up a crack in a non-critical plywood bulkhead. I'm letting it set up for a few more days before my plan to rough it up and glass over it with cloth and resin. I went to the only local marine supply shop on the island yesterday looking for a small fiberglass kit. They had one in stock, a pint of resin, tube of hardener, square yard of cloth, etc. I bought it. Everclear, or Evercoat, something like that.

When I got home I noticed it's polyester resin. All my old surfboard building days and afterwards I always used epoxy resin, so have zilch experience with this stuff.

The smallest amount of epoxy resin I saw at the store was a gallon of West Marine stuff, but I am not really interested in buying that much. Its all expensive here. The kit was almost forty bucks. A half pint should do me for the first coat, with enough left over to add another layer if I feel like it needs it.

I dont get it, why you use 5200 to seal a crack ? better to use epoxy mixed with silica or microballons , Polyester over 5200 is new for me, never try it, but for sure Polyester is not a good adesive over 5200, anyway if the crack is just a scracht in a hiden place and is not structural go ahead,,, but in your shoes i remove the 5200 and do it again properly with epoxy for a last forever repair...
Good luck!

Aloha Canibul,
You mentioned that it is a non-critical bulkhead. Is it something that you see all the time or is it hidden?
Usually 5200 is used as an adhesive bedding compound and not as a gap filler so that's probably why you got a couple of critical posts. I consider 5200 too expensive to use to fill gaps. That doesn't mean you can't do it. If the crack is filled and doesn't need any more reinforcement why not just paint it? If it does need more strength then you'll need to do something different. Once you've got 5200 on something it might be really tough to remove so you might have to bite the bullet and get an epoxy repair kit.
kind regards,

35 years ago I watched a toothless, tobacco chewin' crew from the Louisiana bayou spread drums of 5200 onto a wooden 72 foot shrimper. The wood didn't look great and the seams had opened a little, but she had gotten to the yard on her own bottom and had been in the shrimpin' service for decades. When they were done troweling on the 5200 they let it cure it couple days and started glassing over it with mat and polyester resin. Seeing as how i was almost 30 years old and knew everything there was to know about the fiberglass business, I quietly laughed and walked away. Few days later, i was talking to one of the crew and politely asked how many of these they had done and if they were sure it worked, how long had these repaired boats been in service, etc. He replied, after a large spit of chaw, "bout a hunnert" and the oldest was about ten years. I have since seen a couple of these damn things, after 35 years, still in service!!!! So yeah, I guess you can put poly over 5200. And, turns out, after almost 50 years in the business, what I've learned is that I'm still learning!!!!